The present invention relates to a mold clamping device for a pressing machine in which a plate material is subjected to bending or other working between upper and lower mold sections clamped against each other.
Various forms of pressing machines for bending plate materials are known. Where a plate material is to be bent by a blade for upper and lower molds being clamped against each other, the blade tends to exert a large force to the plate material, leading to a deviation therein. If the upper mold is pressed against the lower mold by a linearly straight force from a hydraulic cylinder or from a simple conversion of the rotation of a motor, a very large power is required. The main body of the pressing machine supporting these power sources will be subjected to overload.
The inventors have found the technical problems inherent in the proposed mold clamping device, improved it to overcome the problems and came up with the present invention.
First of all, the technical problems raised by our previously proposed mold clamping device will be described in connection with FIG. 5.
The mold clamping device shown in FIG. 5 has a main frame 50 which comprises a lower frame portion 51 having a lower mold 53 and an upper frame portion 52 having an upper mold 54. The lower and upper frame portions 51, 52 are pivotally connected to each other at the rearward ends through a pivot shaft 55. The lower and upper frame portions 51, 52 are also connected to each other at their forward ends through an arch-type thick spring 56. Thus, the structure thereof necessarily becomes complicated. And yet, the arch-type thick spring 56 causes the whole height of the mold clamping device to increase since the arch-type thick spring 56 must be curved rearwardly to an increased extent such that it will not interfere the plate bending action in the pressing machine. In addition, the arch-type thick spring 56 is a bulky metal working matter, for example, having a thickness of 15 cm and a width of 20 cm in cross-section since it must provide a strong and instantaneous force. This not only increases the manufacturing cost, but also requires a huge driving energy for moving the upper frame portion together with the arch-type thick spring. This additionally raises a poor response in power transmission.
A crank mechanism 58 for driving the arch-type thick spring 56 is provided on the upper frame portion at the top thereof. The upper end of the arch-type thick spring 56 is connected with a crank pin 60 in the crank mechanism 58 while the bottom end thereof is supported by another pin 61.
In such an arrangement, the crank mechanism will provide an opening force for elongating the arch-type thick spring. A closing spring force for depressing a holding ram 62 is created from a reactive force associated with the resilient opening deformation of the arch-type thick spring. However, the arch-type thick spring 56 highly restricts the amount of resilient deformation. It is thus difficult that the clamping force is regulated into the desired level.
For such a reason, the length of the arch-type thick spring needs to be increased. As a result, its point of action 60 will positioned upward from the upper frame portion, so that a forward thrusting force P will act on the holding ram 62 in association with the structure of opening/closing frame 1. This may destabilize the engagement between the upper and lower molds.
Even in the regulation of clamping force, the mold clamping device hardly accommodates the change in the set height of the holding ram, for example, due to mold exchange or corrective grinding since the amount of resilient opening deformation in the arch-type thick spring is smaller, that is, the range of track using the clamping force in the crank pin 60 is smaller.
Moreover, the upper mold must be replaced by another mold when it is to change the plate bending process or the shape of the product. This is very troublesome in operation. To facilitate such an operation, a shaped mold such as a rounded mold 21 may be placed on the upper mold 54. To accomplish this, the height of the holding ram 52 must greatly be changed. However, this operation is very difficult since the holding ram 62 must be moved up and down with the opening/closing motion of the upper frame portion 52, rather than the independent movement of the holding ram 62 itself. In addition, when any shaped mold is mounted on the upper mold 54, a deviation tends to create in the engagement between the upper and lower molds 54, 53 since the upper mold 54 mounted on the holding ram 62 will be moved up and down along a circular track R about the pivot pin 55. To overcome such a deviation, the mold clamping device must be designed and regulated in an extremely delicate manner.
An object of the present invention is therefore to provide a mold clamping device for a plate bending pressing machine, which is of a simple structure with a low manufacturing cost, is superior in saving the energy, provides an accurate and stable clamping operation and has an improved operability.
To this end, the present invention provides a mold clamping device comprised in a plate bending pressing machine including a main frame having an integral structure, upper and lower molds between which a plate material is inserted and the upper mold being pressed against the lower mold to bend the plate material, comprising:
a vertically movable holding ram for holding the upper mold;
at least a pair of left and right arch-type thick springs vertically mounted on an upper half of the main frame;
a crank mechanism being connected with each of upper portions of the arch-type thick springs through a crank pin and creating a rotational force at a position near a dead center on each of the crank pins,
wherein the rotational force of the crank mechanism compresses the arch-type thick springs to produce a resilient force which presses the upper mold against the lower mold.
In such an arrangement, the crank mechanism is actuated to lift the holding ram together with the arch-type thick springs for opening the upper mold. When the upper mold is opened, a plate material is inserted into the machine. The crank mechanism is then re-actuated to lower the holding ram together with the arch-type thick springs for closing the upper mold. At this point, the arch-type thick springs are largely compressed near the dead centers of the crank pins. Therefore, the plate material may be clamped against the lower mold under an appropriate resilient force with flexibility but without being stiffened. Furthermore, since the holding ram is moved in the vertical direction, the upper mold may be accurately and stably engaged with the lower mold regardless of the height thereof.
The pressing machine of the present invention may comprise a C-shaped main frame having an opening, upper and lower molds located within the opening, a blade drive located in the lower half of the main frame and a blade for bending the plate material clamped between the upper and lower molds when driven. In such an arrangement, when the blade is to bend the plate material, the upper mold is clamped by the resilient force from the arch-type thick springs. Thus, a little deviation is created in the plate material. This provides stable bending with improved dimensional precision. The holding ram may include a bracket extending therefrom. If a displacement crank mechanism using the pins connecting the arch-type thick springs to the holding ram as crank pins is mounted on the bracket, the upper mold can be mounted and used when the holding ram is lifted by the displacement crank mechanism. The holding ram is slidably and vertically held on guide rails on the main frame through guide blocks.
According to the present invention, the upper portion of the integral main frame structure can include relatively small arch-type thick springs. Thus, the present invention may provide a simple and inexpensive structure with less energy. Since the crank mechanism vertically drives the upper mold holding ram through the arch-type thick springs to compress them, the compressed arch-type thick springs creating the resilient force for clamping the molds, the clamping process can be quickly carried out with accuracy and stability. In addition, the displacement crank mechanism using the pins connecting the arch-type thick springs to the holding ram as crank pins can provide advantages in that the machine can properly accommodate changes and modifications of the mold and improve the operability. In the aforementioned structure of the crank mechanism, the displacement crank mechanism located on the bottom of the arch-type thick springs may be used for clamping the upper mold while the crank mechanism on the top end thereof may be used to displace the upper mold for mounting a shaped mold thereon.